1. Department of Semiconductor Systems Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Korea
2. Center for Spintronics, Korea Institute of Science and Technology (KIST), 5, Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, Korea
3. School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul 02841, Korea
4. Department of Materials Science and Engineering, University of Texas, Dallas, Richardson, Texas 75080-3021, United States